Cellular cholesterol homeostasis is maintained by rigorous control of both biosynthesis and uptake. The nature and the mode of action of endogenous regulators that modulate this critical balance are the subject of investigation in this proposal. In Specific Aims #1, we describe experiments designed to elucidate the molecular mechanism of action of one class of natural compounds called epoxysterols. These compounds are the by-products of cellular cholesterol biosynthesis and appear to act in a unique way to inhibit the rate-limiting steps of synthesis and influx of cholesterol. We propose to study the coordinate control of several proteins of isoprenoid metabolism by epoxysterols at the levels of activity, gene transcription, message translation and protein degradation in both hepatic and non-hepatic cells nad also in mutant cells known to be resistant to regulation by oxysterols. In Specific Aim #2, we propose to apply the above analytical techniques to study the mechanisms and the role of non-sterol derivatives of mevalonate metabolism in the regulation of cholesterogenic enzymes. Through the use of mutant cell lines and specific inhibitors, we will attempt to sort the independent and we will also compare non-sterol regulation in proliferating fibro-blasts with quiescent hepatic cells to study cell-type specific regulation. In Specific Aim #3, we propose to examine the regulation of cholesterol uptake mediated by the low-density lipoprotein receptor by epoxysterols as well as non-sterol products of mevalonate in hepatic cells from three different sources. These studies may help shed light on the mechanisms of action of novel classes of regulators of cellular levels of cholesterol, molecule of high current interest.